1. Field of the Invention
The present invention relates to mills for pulverizing coal and other fuels, and in particular, to mills in which the dynamic or operating elements are assembled to form a compact, unitized package that can quickly and easily be slipped into and out of the basic mill structure.
2. Related Art
The prior art contains multifarious instances of dual counter-rotating rotor mills (with dual shafts and drives) but exhibiting no features for rapid or easy disassembly, which is desirable for economic reasons. In this category fall Evans (2,361,278), Meger et al (3,047,343), Noe (3,411,724), Hint (3,497,144), Smith (3,817,460), Benedikter (3,894,695), Brown (4,355,586), and Muschenborn et al (4,522,342).
Even in the case of an earlier Benedikter patent (3,823,919) concerned with solving the problem of readily removing buildups of process material from interior mill surfaces, the concept is to transport the buildup out of the mill rather than to effect economic disassembly or replacement of critical components. Parmele (3,317,975) approaches the problem by removing one rotor from the other by means of a trolley. That type of solution nevertheless requires loss of operating time. The present invention provides significant economic advantages to mill owners such as utilities for whom large dollar benefits can result from avoiding unplanned downtime otherwise occurring with mills inherently impossible to service fully with rapidity.
The invention depends on a shaft-within-shaft drive arrangement. Durek (4,406,409) employs two hollow shafts riding on a common fixed shaft. Pallmann (3,549,093) use a shaft-within-shaft arrangement. Neither of these, however, offers any component replacement economies.
An object of this invention is to improve the size reduction technology for coal, minerals (including ores, compounds, and elements), biomass waste, and other materials.
A further object of this invention is to provide efficient means for quickly returning service-critical mills to full service by replacing one unitized package (or module) containing both milling components and drive motors.
Another object of the invention is to provide capability to service worn milling components with activity that does not interrupt mill production time.
It is yet another object of this invention to improve the technology of grinding, such as with coal fuel of micronic size particles that can be burned much like oil or gas in boilers using air to blow fuel to burners for combustion, substantially reducing nitrous oxide emissions due to small, average coal particle sizes.
Current technology for utility coal grinding generally employs rollers or balls to crush coal in a rotating bowl. Coal feeds in at a controlled rate; and powdered product coal is removed by passing high velocity air through the mill and picking up coal fines as they pass from the rolling zone. Generally, the stream of air bearing the coal dust passes through a classifier that separates out oversize particles and sends them back through the rolling zone for further grinding. By necessity these mills are heavily built to withstand the heavy forces applied.
Maintenance in these bowl mills is expensive, and breakdowns or even planned maintenance is time-consuming. Mills can be down for days or even weeks. The mill in accordance with the present invention is totally different in concept. It depends on high velocity impacting and abrasion created by the use of high speed multiple-ring rotors that are juxtaposed face to face such that process material is thrown by centrifugal force from the rings of one rotor to the rings or the other (counter-rotating) rotor. From the resulting labyrinthine passages, process material exits the rotor set reduced to a very fine state. Various configurations of ring structures produce different fineness levels of reduction. Some of these constructions are subjects of other patent applications.
In the present invention, the use of speed to impart destructive forces in the reduction process, rather than force applied by heavy elements, permits the use of lighter structure that can be configured into a simpler and more compact unit. The motors are directly coupled to their respective rotor assemblies. There are no heavy gearboxes necessary for reducing high motor speeds to slow bowl or roller rotating speeds. The other attendant structures can be either eliminated or lightened in weight considerably and arranged for rapid replacement.
The present invention provides large economic advantages for mill owners like utilities for which large dollar benefits or penalties can result from unplanned downtime with mills inherently impossible to service fully with rapidity.
Other objects, features, and advantages of the present invention will be apparent to those skilled in the art upon a reading of this specification including the accompanying drawings.